User-Interface Approaches :

Layered.

Emulated.

Layered Approach :

This is the most commonly used approach.
Product is just a layer on top of the native GUI application program.
It is more likely to get the native look and feel correct on all supported platform.
Vendor produces less software since the PIGUI piggybacks on the native platform products, therefore tends to be less expensive.
Example products that use this approach are: C++/VIEWS, OpenUI, StarView, WNDX, XVT, ZINC.

The PIGUI software provides the look and feel.
User can test the look and feel of the UI on the target platform before porting it to the target platform. For example, if you are developing the application in MSWindow and your target platform is the Mac, you can see what the GUI would look like on the Mac from MSWindow without first porting it to the Mac.
Vendor produces more software, products therefore tends to be more expensive.
User don't have to purchase or learn the toolkits for the GUI on the target platform.
Galaxy, OpenInterface, Visual Works are some products that use this approach.

Emulating the original platform's API on the new platform.
This approach is an up and comming approach.
The problem with this approach is that different API do not map precisely to one another.
Some example products are : MainWin, MacWell, Win/U and Mewel.